My guess is that this is loading a variable called SPIODAT (SPI Output DATa, maybe?) with some number, then calling a function SPI to send it out the SPI pin. The CSN is some type of control. You will have to find out how your C compiler deals with bits in order to do the SBIT part.

The RET is handled by C's end function:
void myFunc(void)
{
/* Do nothing */
} <- This will cause the compiler to generate the RTS command.